Mr. H. Seeley on tv:o new Plesiosaurs from the Lias. 357 



rugose border is present in the whole series of vertebrae ; and 

 adds that the whole of the limb-bones, including the phalanges, 

 as well as other parts of the skeleton, are more or less rugose. 



Our animal appears to have been mature, though not aged : 

 what remains of it rests on the right side. 



In P. rugosus there are said to be about 35 or 36 cervical 

 vertebrpe : but the only one described is supposed to be the 15th ; 

 and plate xiv. of the Monograph of Lias Plesiosaurs shows it 

 to be about two-thirds the size of those at the base of the neck. 

 Now, as the vertebrae from the base of the neck in our specimen 

 arc shorter than that from the middle of the neck in P. rugosus, 

 and as the lower dorsal vertebrae want the rugose margins, there 

 is strong probability that the species is new. 



It would naturally seem a portion of a vertebral theory that, just 

 as certain vertebrae between the back and tail are reckoned pelvic 

 or sacral, so there might be some between the neck and back simi- 

 larly modified by relations to the fore limbs, which could only be 

 reckoned pectoral. At present there is no possible means re- 

 cognized for determining in Plesiosaurus the limits of dorsal 

 and cervical vertebrae. Prof. Owen's dictum in 1839 is different 

 from Prof. Huxley's in 1858; and Prof. Owen, in 1861, lays 

 down a different law to that given by him in 1865. Indeed 

 the conditions seem to be different in nearly every species, the 

 difficulty being to know what to do with those vertebrae where 

 the costal surface is passing from the centrum to the neural 

 arch, in which the ribs are extremely variable. The confu- 

 sion which has hitherto marked descriptions of this region 

 might be easily avoided by reckoning such vertebrae pectoral, 

 and counting them separately, while it would introduce a new 

 character whereby to distinguish species. 



There is the usual difficulty in this species in determining 

 the first dorsal. As in P. eleutheraxon, the costal surface in 

 the last few cervicals enlarges, becoming more elliptical, and 

 then in those vertebrae where it is partly formed by the neura- 

 pophysis becomes smaller, and enlarges again in the back. 



The first three vcrtebra3 are clearly cervical, and the first of 

 them measures — 



From base of centrum to top of neural spine. . 4^ inches. 



The height of the centrum is 1|- inch. 



The length of the centrum is l-f „ 



but it is longer at the top and at the bottom. 



Length through the centre 1 „ 



\ From praszygapophysis to postzygapophysis is 2f inches. 



The articular surfaces of the centrum are flat at the outer 

 part, and rather deeply cupped in the centre, where there is a 



